1. Essence of what it means to be human, in a human body. No soul equals zombie or vampire or something else not entirely alive and decidedly unpleasant. (Unless you are The Master for some reason, in which case you get the custom made, one-of-a-kind Master Soul.)

2. The thing that makes it possible for beings to give a crap about good and evil. A sense of morality.

3. An torturing entity not banned by the Geneva Convention whose pain inducement potential registers an 11 on the Guilt-O-ScaleTM. It can turn flighty when distracted by happiness.

4. A valued possession used to justify caring about another being.

5. A catalyst for a psychotic break.

6. The Swiss Army Knife of writer's MacGuffins.

7. A magnet for endless cud chewing debate.

Origin: Early Buffyverse

Angel: ...The elders conjured the perfect punishment for me. They restored my soul. ...When you become a vampire the demon takes your body, but it doesn't get your soul. That's gone! No conscience, no remorse... It's an easy way to live. You have no idea what it's like to have done the things I've done... and to care.